The first spread of Christianity
For thirty years Paul, a disciple of Jesus, traveled around 10,000 miles spreading the gospel throughout Rome. Paul was the Romans first taste of the gospel, and the early idea of Christianity is what shaped, and is still shaping Christianity today.
The Apostolic period
The Apostolic period was during the time that all of Jesus's apostles were still alive. This period ranged from the time that Jesus was alive, and until the last apostle, John, died.
Throughout this period, the Apostles of Jesus, and even some family members of Jesus would go around spreading the gospel just like Jesus had commanded that they do. Some apostles, like Paul traveled very far to spread the gospel.
The first Christians of the time were actually Jewish. At the time Judaism was the first true "form' or Christianity. The first Apostles of Jesus came from Jewish backgrounds.
Judaism back then compared to Judaism now is different. Jesus was referred to as the King of the Jews. Back then his followers were mainly Jewish, but most Jews in this time period do not believe that Jesus was the son of God, and they are still waiting for the Son of God to come to earth for the first time. Christians today believe that Jesus was the Son of God and already came to earth the first time, and the next time he comes back it will be for the Rapture that is talked about in the Book of Revelations.
A new beginning
The Great Awakening made it's way through the protestants in American Colonies brought back traditional Christianity. At this point a lot of the common ways of teaching Christianity began to change. A lot of the common baptist teachings that are being used today were developed back then. The first Great Awakening also helped develop democratic teachings and that information coming from the bible should be shared and should not be biased.
The Mystery religions were very old religions that were practiced throughout the Roman empire. These religions were derived from two main sources which was Egyptian mythology and Greek mythology.
The reason that Christianity can be "Considered" a "Mystery religion" is because a lot of the common religions had something to do with a God or Goddess dying, but was restored.
Eventually mystery religions died out, but Christianity stood strong.